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Environmental Applications of Chiral High-Performance Liquid Chromatography 1W-Y

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Environmental Applications of Chiral High-Performance Liquid Chromatography 1W-Y ENVIRONMENTAL APPLICATIONS OF CHIRAL HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY 1W-Y. Lee and 2J.M. Salvador 1Environmental Science and Engineering, 2Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968; Phone: (915) 747-5704; FAX: (915) 747-5748. ABSTRACT A commercial pharmaceutical analysis chiral method development kit (Chirex Column Kit A, Phenomenex) was used to analyze six pesticide stereoisomer mixtures. The pesticides were selected from the 1994-1995 National Water Quality Assessment Program (NAWQA) covering New Mexico and Texas. Three stereoisomer mixtures were separable on three different columns. The instrumental detection limit and detection linear range of chiral high performance liquid chromatography (HPLC) of these analytes were determined. The efficient extraction (> 90%) from soil of one chiral mixture was demonstrated. The resolution of a shorter kit column versus an analytical column was also compared. Key words: chiral, chromatography, HPLC, pesticide, stereoisomer INTRODUCTION limited to only one enantiomer, with the activity More than 1.1 billion pounds of pesticides of the other enantiomer being less effective, are used in the United States each year to inactive, or different (Ariens et al., 1988; Buser control weeds, insects, and pests in agricultural and Müller, 1995; Venis, 1982; Faller et al., and non-agricultural settings (Barbash and 1991; Renner, 1996). For example, some Resek, 1996). The increasing use of synthetic bacteria only degrade one enantiomer of organic pesticides raises many concerns about Mecoprop (R) (Tedd et al., 1994; Ludwig et potential adverse effects on the environment and al., 1992; Falconer et al., 1995; Iwata et al., human health, and increases the importance of 1998). The toxicity of the (R)-enantiomer of analyzing pesticides in the soil, water, and air Fonofos, an organophosphorus pesticide, in (Barceló, 1991; Müller and Buser, 1995). To mice is greater than the toxicity of its mirror assess the environmental impact of pesticides image, (S)-Fonofos (Kurihara et al., 1997). and reduce the risk of public exposure, all Ignoring the existence of enantiomers can lead aspects of pesticide chemistry need to be well to incorrect toxicological, distribution, and understood and studied. degradation data. Ongoing research is aimed at Approximately a quarter of the hundreds identifying chiral pesticides, determining their of pesticides in use are asymmetric or chiral distribution, and measuring their degradation. (Milne, 1995). Chiral molecules may exist as Chromatography is the most common mixtures of non-superposable mirror images or method for enantiomeric analysis. Gas chroma- enantiomers. Enantiomers have different tography (GC) is the primary method used in chemical and physical properties in asymmetric pesticide analyses (Grosser et al., 1993). environments. The desired biological activity of However, high-performance liquid chromatog- a chiral pesticide enantiomer mixture may be raphy (HPLC) is more suitable for larger, non- 36 Proceedings of the 2000 Conference on Hazardous Waste Research cides are chiral. The stereoisomer mixtures of Fonofos, Malathion, Metolachlor, Napropamide, Permethrin, and Propargite are shown in Figure 1. Although the enantiomers of chiral pesti- cides may have different biological properties in the environment, the NAWQA study completely ignored stereochemistry. This may be due to either a lack of knowledge of the potential problem or a lack in methods or technology to analyze chiral pesticides. Current U.S. EPA methods (507 and 508) use GC to detect some of these pesticides in drinking water but do not separate stereoisomers of chiral pesticides. For Figure 1. Structures of the stereoisomers of our work we selected six chiral pesticides from the six chiral pesticides from the NAWQA the NAWQA study to demonstrate the applica- study of Texas and New Mexico. tion of chiral HPLC in pesticide analysis. volatile, polar, and thermally labile pesticides Though one of the seven chiral pesticides (Buser and Müller, 1995; Farran et al., 1996). studied in NAWQA, α-Lindane was excluded Also, HPLC can tolerate large-volume injec- because our HPLC detector depends on UV tions of aqueous samples, rendering it ideal for (ultra violet-visible) absorption. screening water samples (Grosser et al., 1993). Since no single chromatography column In fact, the number of official U.S. Environmen- can separate all compounds, to find suitable tal Protection Agency (EPA) methods using stationary phases and separation conditions can HPLC grew to more than 40 approved and be expensive and time consuming. Thus we draft methods by 1993 (Grosser et al., 1993). used the shorter columns of a commercial chiral The Rio Grande Valley study unit of the HPLC method development kit to economically U.S. Geological Survey National Water Quality survey the efficiency of normal and reverse- Assessment Program (NAWQA) conducted a phase systems. The survey was used to guide study of the occurrence and distribution of the purchase of more expensive and longer pesticides in the surface water of our region of analytical columns. New Mexico and Texas (Healy, 1996). The We used Soxhlet extraction (EPA Solid study selected 40 pesticides and their metabo- Waste Test Methods, SW-846 Method 3540C lites on the basis of national usage, and the (Understanding Environmental Methods, CD- development and cost-effectiveness of analytical ROM, 1998) to demonstrate the recovery and procedures. Seven of the 40 selected pesti- analysis of a chiral pesticide mixture from soil Proceedings of the 2000 Conference on Hazardous Waste Research 37 samples. This method is applicable to the ses were performed using Winner on Windows quantitative extraction of nonvolatile and (WOW) software. semivolatile organic compounds from solids The Chirex™ Chiral Method Develop- such as soil, relatively dry sludge, and solid ment Kit A (Phenomenex, Inc., Torrance, waste in preparation for a variety of chromato- Calif.) used in this work contained five 50 x 3.2 graphic procedures. Soxhlet extraction uses mm normal and reverse-phase HPLC columns: relatively inexpensive glassware, once loaded Chirex™ 3001, 3005, 3010, 3014, and 3020. requires little hands-on manipulation, and An additional Chirex™ 3001 analytical size provides efficient extraction. However, it uses (250 x 4.6 mm) column was also purchased. fairly large volumes of solvent and is rather time All HPLC injections consisted of 1µL into a consuming (16 to 24 hours). Combined with 20µL loop. SW-846 Method 3540C, the chiral HPLC Extraction and Concentration analysis methods presented are a beginning The Soxhlet extractor (Kimax, Fisher to study the different bioactivity, biodegra- Science) had a 40 mm ID with a 500-mL round dation, accumulation, and distribution of bottom flask. Soil samples were contained in pesticide enantiomer pairs in soil, surface cellulose thimbles (Whatman, Fisher Science). water, and groundwater. A Kuderna-Danish (K-D) apparatus was EXPERIMENTAL used to concentrate extracts. It included a 10- mL graduated concentrator tube, a 500-mL Materials evaporation flask attached to the concentrator Racemates (1:1 enantiomer mixtures) of tube with springs, and a Snyder column. A Metolachlor, Napropamide, and Permethrin solvent vapor recovery system (Kontes, Fisher were kindly provided by Norvatis (Greens- Science) was attached to the top of the Snyder boro, N.C.) and Zeneca (Richmond, Calif.). column to reduce emissions and minimize waste. Mixtures of Fonofos, Propargite, and Malathion were purchased from Chem Service Methods (West Chester, PA.). HPLC grade solvents Preparation of standards (isopropanol, hexane, acetonitrile, and water) To prepare stock standard solutions, 25 were obtained from Aldrich Chemical Co., mg of each pesticide were accurately weighed Milwaukee, Wis. All materials were used and dissolved in an appropriate solvent to make without further purification. a 25-mL solution. The purity of each pesticide Chromatography was 96% or greater; therefore, weights were Liquid chromatography was performed used without correction to calculate the concen- using a Spectra-Physics HPLC system consist- tration of each stock standard solution. Methyl- ing of a P2000 gradient pump and a UV2000 ene chloride was used to dissolve detector. Data acquisition, storage, and analy- Napropamide, and hexane was used to dissolve 38 Proceedings of the 2000 Conference on Hazardous Waste Research ROM, 1998). Soil was collected, passed through a 100-mesh screen, washed with water and methanol, and air dried in a hood overnight. A 10-g soil sample was then accurately weighed and put in an oven overnight at 105 °C. The percent dry weight was determined by the following equation: g of dry sample % dry weight=X 100% g of sample The oven-dried soil sample and 10 g of anhydrous sodium sulfate were mixed and put into a thimble. Soxhlet extraction of the soil sample was then performed as outlined in Figure 2. An outlined procedure of Soxhlet Figure 2. extraction of soil samples. For reasons to be discussed later, Napropamide was chosen for soil sample Fonofos, Malathion, Metolachlor, Permethrin, recovery analysis. A 50-mL spiking standard and Propargite. In the cases that separation of solution was prepared by dissolving 50 mg of isomers was not observed by HPLC, no pure Napropamide in methanol. The stock further standard solutions were made. Other- standard solution was stored in a Teflon-sealed wise 10 mL of standard solutions of each container at 4 °C. pesticide
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